Commercial vehicles may utilize an on-board compressed air system to supply compressed air for operating air brakes, lift axles, air suspension, air powered accessories and perform other functions. The compressed air often contains moisture that must be removed to avoid the build-up of water in the air lines. Water in the air lines may cause problems, such as freeze-up in cold conditions, that may lead to system failure. Known air drying systems dry the compressed air by flowing it through a desiccant material to absorb the moisture. The desiccant material may be housed in a spin-on cartridge or canister that attaches to an air dryer body or manifold. Typical air dryers utilize one desiccant cartridge to dry the air. When the vehicle is not demanding compressed air, the system will regenerate the desiccant by reverse flowing a small percentage of the dried air back through the desiccant to absorb and discard some of the moisture collected.
In some vehicle applications, however, such as vehicles with multiple lift axles, the demand for compressed air, and thus air drying, may exceed the capacity of a known single desiccant cartridge system. When the drying capacity of the system is exceeded, the air dryer system becomes overwhelmed, which may result in moisture passing into the air brake system and the desiccant breaking down